Method of correlating subsurface strata



Janyll, 1938. H. N. HERRICK 2 1 METHOD OF CORRELATING SUBSURFACE STRATA Filed Ja n. 25, 1937 Inventor HENRY /v. HERR/CK I Aifoz nqy Patented Jan. 11, 1938 UNITED STATES PATENT OFFICE ATA Henry N. Herrick, Berkeley, Calif., assignor to Standard Oil Company of California, San

Francisco, Calif., a corporation of Delaware Application January 25, 1937, Serial No. 122,187

4 Claims.

This invention relates to improvements in methods of correlating subsurface strata, as they may be encountered by'the drilling of one or more wells, by a comparison of distinctive and 5 determinable characteristics of samples of the said strata, and particularly refers to the correlation of strata by a comparison of the magnetic susceptibilities of the same.

In the drilling of wells, especially in drilling for oil or gas, it is very desirable to obtain information about the sequence, existence, and position of the geological strata penetrated by the drill, and'to be able to correlate those data with similar information obtained from adjoining and distant wells. Successful correlations give an accurate and very valuable indication as to subsurface conditions at points between the wells or bore holes, as well as in the surrounding areas.

Attempts have been made to correlate strata by microscopic analysis and comparison of the minute fossils they contain, as well as proportions of certain heavy minerals, which form so-called marker beds. These methods involve a careful preparation and microscopic examination of 5 samples and not only require skilled operators and elaborate equipment, but a long and tedious period of time, so that but few determinations can be made in a day and at a considerable cost. Also a large proportion of strata have no dis- 3() tinctive marker beds or forams, so that many feet of cores must be laboriously examined to find a small portion that may be usable for this purpose.

This invention involves theutilization, for cor- 35 relation purposes, of a hitherto unused property of a sample of subsurface strata, namely, its magnetic susceptibility, and has been found to be of great commercial value in such correlation procedure.

40 It is. appreciated that the magnetic properties of cores and samples of cuttings from subsurface strata have been determined, and attempts have been made to determine the relationship between such measurements and the results of 45 magnetic field intensity surveys carried out at the surface of the earth to locate tectonic structures which may be favorable to the accumulation of oil, gas, salt, metallic and particularly ferrous ores and the like. Such are discussed in some de- -50 tail in the publication Geophysical Prospecting published in 1932 by the American Institute of Mining and Metallurgical Engineers, particularly in the section entitled A Method for Determining Magnetic Susceptibility of Core Samples.

55 Reference is therefore made to that publication for the various well known forms of apparatus and technique of operation to perform such measurements, as will be supplemented by a further description of a preferred mode of operation and equipment therefor in the following specification. 5 It is an object of this invention to provide an improved method of locating,- identifying, and

correlating subsurface formations so that they .may be distinguished and identified at a plurality of points at which they may be intersected by 10 well bores from which samples may be retrieved.

Another objectis to provide an improved method of correlating well logs and cores from adjoining wells by a comparison of the magnetic susceptibilities of'the said cores. 15.

- matical computations.

These and other objects and advantages of this invention will be more fully apparent from the following description'of the method and from the accompanying drawing, which forms a part of this specification and illustrates a preferred form of apparatus by means of which the step of measuring of the relative magnetic susceptibilities of the samples may be carried out.-

In the drawing, Figure 1 is a vertical and part sectional view of an apparatus which may be used to measure magnetic susceptibility of a sample of core material.

Figure 2 is a. horizontal sectional view on line II-II of Figure 1, and illustrates the preferred 40 relation between the normal magnetic axis of the magnetic system and the sample holder.

Figure 3 shows a sample chart or log of a well bore on which has been superimposed a record of the magnetic susceptibility of the strata traversed, and indicating, as at A, B, C, etc., the distinguishing strata or magnetic marker beds that may be correlated with similar data from another well.

Referring to the drawing, and particularly to Figure 1, a torsion type magnetometer or magnetic balance isgenerally indicated at Ill, and comprises a brass or other non-magnetic metal case I I in which is suspended an astatic magnetic system l2, by means of a torsion wire l3, which may be adjusted by head ll. A mirror I! is secured to the magnetic system and is adapted to cooperate as shown with a light beam from lamp l6 and conventional arcuate scale I! to indicate the deflection of the magnetic system.

Magnetic system I2 is preferably supported by and enclosed in a heavy steel or iron shield IS in which are suitable openings i9 and 23, for the light beam from the mirror I5 and for the manipulation of the sample carrier 2i, respectively. The sample 22 of the core or other sample of the material whose magnetic susceptibility is to be determined is preferably pulverized in a nonmagnetic vessel, such as a porcelain mortar, to pass a flne screen, say 40 mesh. A precisely measured amount, say 30-grams, is placed in a glass tube 23 of about 21 mm. bore and 11.5 cm. length, and the ends are closed by corks 2|. 1

The holder or carrier 2i for the sampleis preferably formed of wood or other non-magneticmaterial and maybe in the form of a v block as shown. It is supported on a non-magnetic shaft 25, secured against rotation as by a spline or keyway 23 and is adapted to be raised or lowered shaft 25 in the desired position, while a scale 29 and pointer 30 may be calibrated to indicate the spacing between the sample 22 and the magnetic system l2, for a purpose which will be explained below.

The magnetometer may be calibrated to give susceptibility in terms of scale deflection in centimeters on scale I! by the use of glass tubes 23 filled with standard solutions of ferric chloride (FeCla) of known susceptibility. It has been found convenient to use four samples; 100% crystalline, and 75%, 50% and 25% solutions, having a susceptibility of 260x 190x 10- 130X10-,

and 65x10-, respectively.

1 Some samples of subsurface strata possess a markedly high degree of susceptibility, over il0 10-, while some may be as low as 20x 10- so it is desirable to calibrate the light beam scale l'l accurately with difierent spacings between the calibrating FeCl: sample and the magnetic system l2, as indicated by the scale 29 and pointer 30. It will be found that the deflection of the beam on scale I! will be substantially inversely proportional to the square of the distance between the sample and the magnetic system.

In operation, the cores or other samples of subsurface strata obtained during the drilling of a well, or thereafter, by well known methods, are carefully pulverized, screened, weighed and placed in tube 23, which is raised to a known distance fromthe magnetic system l2 by shaft and carrier 2|. The sample will have no polarity due.

to its heterogeneous arrangement, even though some of its components may be individually polarized. It will, however, be found to be measurably susceptible to the effect of the external magnetic fleld of the-externally shielded astatic magnetic system i2, and will produce a reaction on that system that will be indicated on scale l1 'bythe deflection of mirror 15 and the light beam from lamp l6.

The values of magnetic susceptibility thus found may be, illustrated or plotted graphically as is indicated on chart 3| of Figure 3, at the corresponding depth at which the samples were found in the earth, as shown respectively by the scales 32 and 33. The graph 34 thus formed is the flnal product of the steps or operations thus far performed. It will be noted that certain strata will have definite and recognizable characteristic magnetic susceptibility values, A, B,-C, .F, and, from their distinctive grouping; position, and magnitude, may be easily identified on the similar graphs made from samples from adjoining and even somewhat distant well bores. From these comparisons, valuable data and inferences may be obtained regarding the slope or dip of certain strata, their presence or absence, thickness, relative depth, and many other types of information that are of interest to those skilled in this art.

It will, therefore, be appreciated that a new and valuable method of correlating subsurface strata and formations has been developed, utilizing the peculiar and distinctively characteristic values of magnetic susceptibility of samples or groups of samples obtained from a well bore to identify formations and materials having otherwise indistinguishable characteristics. Although but one type of apparatus is disclosed. as bein useful for carrying out the measuring 'step of this method, it will be appreciated that many other types are available, as has been referred to above. It will -also be recognized that actual plotting of the values need not be carried out, as a mere inspection of the tabulated quantities will often give the required information and correlation.

I claim:

1. The method of correlating subsurface strata which comprises the steps of measuring the magnetic susceptibility of a succession of samples of suburface strata from a bore hole intersecting the same, measuring the magnetic susceptibility of a succession of samples of subsurface strata from a] second bore hole at a distance from said first borehole and comparing the results so found to identify similar characteristicsof the strata in or adjacent the said bore holes.

2. The method of correlating subsurface'strata which comprises the'stepsof measuring the magnetic susceptibility and the depth of a succession "of samples of subsurface strata from a bore hole intersecting the same, measuring the. magnetic susceptibility and the depth of a succession of samples of subsurface strata from a second bore hole at a distance from said first bore hole, and

comparing said magnetic values and said depth values to determine the characteristics of the strata in or adjacent said bore holes.

3. The method of correlating subsurface strata lacking in distinctive physical components or characteristics which comprises the steps of measuring the magnetic characteristics of consecutive samples from a bore hole, measuring the magnetic characteristics'of consecutive samples from a second bore hole and comparing the said characteristics to identify strata intersected by both of said bore holes.

4. The method of correlating subsurface strata which comprises, the steps of measuring the magnetic properties of samples from a series of otherwise unidentifiable strata anil comparing the values thus-found with values of magnetic properties of samples. from a second series of strata so that strata common to both series may be identified.

- HENRY N. HERRIOK. 

